1. Field of the Invention
This invention relates to the three dimensional imaging of objects using a single pulse of energy.
2. Description of the Prior Art
A previous patent application Ser. No. 799,424 describes a similar machine. However in that application the reconstruction process was carried out in two geometrical steps. In the first step the image was resolved with respect to each of two angular directions originating at the transmitting tranducer. In the second step the ranges along each two dimensional angular direction are resolved. This requires a complicated mathematical algorithm to be implemented in the reconstruction hardware, and in addition, mathematical approximations to the wavefield geometry may be required.
The previous approach essentially uses spherical coordinates originating at the center of the transmitting transducer. This yields a lateral resolution that is very small near the the transducer but grows larger as the distance from the transducer increases. However, a large aperture formed by a sparse array is capable of producing relatively uniform resolution for ranges equal to several diameters of the array. The previous approach shows an intermediate memory called the data memory which is used to store the three dimensional reconstructed field of reflecting objects. This is an intermediate memory since it must be further processed into a tomographic image or a two dimensional view through a three dimensional field of objects. The present invention can reconstruct tomograms or two dimensional views through three dimensional fields directly from the stored time history of the receiver elements. This increases the processing speed and reduces the amount of electronics by eliminating the requirements for an intermediate memory.
The final display will most likely be in rectangular coordinates whereas the previous approach reconstructs in spherical coordinates. This results in complex electronics being required in the implementation of the machine.
The transmitted wavefield from a small transducer emitting a pulse will approximate an expanding sphere at locations several diameters away from the transducer. This must be taken into account by the reconstruction processor or image degradation will occur. The previous approach makes no explicit provision for this and it would be hard to implement in such a two step reconstruction technique. The implementation would require look up tables or computation circuits for each reconstruction point and each receiver element. This would be hard to implement in real time and would use a large amount of electronics.
The present invention uses a round trip time of flight algorithm which automatically takes into account the curved nature of the wavefront propagating away from the transmitter. In addition no Fraunhofer or Fresnel approximations are made since the algorithm is essentially a Huygens wavelet based approach. It requires only the computation of the distance from the transmitter to the reconstruction point and computation of the distance from the reconstruction point to each receiver element.